Transmission line attenuators for high power



March 14, 1967 H. A. WHEELER ETAL 2 Sheets-$heet 1 Filed Dec. 14, 1964LINE COU- PLING ATTENUATOR COU- PLING LINE FIG. 1

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March 14, 1967 H WHEELER ETAL 3,309,634

TRANSMISSION LINE ATTENUATORS FOR HIGH POWER 2 Sheets-Sheet 2 Filed Dec.

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United States Patent 3,309,634 TRANSMISSION LINE A'I'IENUATORS FOR HIGHPOWER Harold A. Wheeler, Great Neck, N.Y., and David S. Lerner, NorthPlainfield, N.J., assignors to Hazeltine Research, Inc., a corporationof Illinois Filed Dec. 14, 1964, Ser. No. 418,003 16 Claims. (Cl.333-81) This invention relates to attenuators and terminations for usewith transmission lines for electromagnetic waves. Attenuator will beused as a generic term with a termination being considered an attenuatorwherein it is intended that all incident power shall be absorbed. Moreparticularly, this invention pertains to high power attenuators usingthe surface resistance of metallic sheets to provide power handlingcapabilities of the order of 100 times as great as prior art attenuatorsof comparable size.

Objects of this invention are to provide attenuators and terminationsfor transmission lines, such as coaxial cable and waveguide, forexample, which permit high power handling capacity for given size orsmall size for given power capacity, or both.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription, taken in connection with the accompanying drawings, and itsscope will be pointed out in the appended claims.

In the drawings,

FIG. 1 shows an attenuator constructed in accordance with the inventionwhich is bisected longitudinally to show inner construction;

FIG. 2 is a cross-sectional view of the complete attenuator of FIG. 1;

FIG. 3 is an oblique sectional view of a small portion of the FIG. 1attenuator;

FIG. 4 is a drawing useful in describing the operation of the FIG. 1attenuator;

FIG. 5 shows a termination with a stepped inner conductor for impedancematching; and

FIGS. 6, 7, and 8 are three views of a waveguide termination usingspaced parallel metal plates to form the side walls of the waveguide.

Referring now to FIG. 1, there is shown a bisected view of an attenuatorcoupled to a coaxial transmission line. The coupling between theattenuator and the coaxial line is shown schematically and any suitableconnector can be used to allow coupling to the connector at the end ofthe transmission line. FIG. 2 is a cross-sectional view of the completeattenuator showing the symmetrical coaxial construction, and FIG. 3 isan expanded, rotated view showing greater detail.

As shown in FIG. 3, this attenuator in accordance with the inventionincludes a plurality of spaced metallic sheets, of which iron washers 10and 12 are typical. The iron washers 10 and 12 are in common electricalcontact at the outer edges due to the inclusion of the narrow conductiveband 14 which bridges the space between the washers 10 and 12. Thecentral portions of the washers 10 and 12 are spaced and noncontacting,and the space between the washers 10 and 12 is shown as being filled bydielectric washer 16. Iron washers 10 and 12 and dielectric washer 16are typical of the large number of similar components included in thecomplete attenuator of FIG. 1.

As further shown in FIG. 3, both the iron washers 10 and 12 and thedielectric washer 16 have central openings in the center portions of thewashers. The holes in all of the washers are centered along the centralaxis of the attenuator, and are therefore coaxial as shown in FIG. 2.Also included is a central conductor 18, positioned coaxially within theholes in the washers to form a symmetrical coaxial transmission line.

Patented Mar. 14, 1967 The additional specific details of constructiondo not directly involve the basic concept of the present invention. Asone example, the conductive band 14 may be a raised rim constructedintegrally with washer 10 or 12 and the attenuator shown in FIG. 1 maybe put into a close-fitting metallic cylinder and end caps screwed ontothis cylinder to hold all the washers together for good electrical andthermal conductivity. The end caps may form part of coaxial connectorsused for connection to a transmission line. Another approach is toassemble the metal washers, such as 10 and 12, and the dielectricwashers, such as 16, on a central mandrel and then cause solder to flowinto the space between the edges of washers 10 and 12 to form bridgingsections, such as 14. Once the present invent-ion is understood, suchdetails can be supplied by workers skilled in this art.

In operation, an electromagnetic wave to be attenuated is coupled intothe end of the FIG. 1 attenuator. As is well known, currents will becaused to flow in both the outer and inner conductors. The presentinvention operates by absorbing power from the current in the outerconductor by converting said current to heat which is dissipated. Aslong as the thickness of the washers 10 and 12 is a few (more than two)times as great as the skin depth of current in the metal in theoperating frequency range, the current in the outer conductor will bemade to follow a long zigzag path to get from one end of the attenuatorto the other. Thus, as indicated in the simplified sketch of FIG. 4,this current must flow along the surface of washer 10, out to the washer14, back in again along the surface of the next washer 12, etc.,throughout the full length of the attenuator.

The total attenuation produced will, of course, depend on the surfaceresistance of the path which the current is made to travel and thecharacteristic impedance of the coaxial transmission line. Therefore, inaccordance with the invention, washers 10 and 12 are most desirablyconstructed of material having high electrical surface resistance and,at the same time, high thermal conductivity. Applicants have discoveredthat iron is an especially useful material for this purpose. Themagnetic properties of iron are helpful in this application because thehigh per- Ineability of iron increases the electrical surface resistanceof the washer, but not the thermal resistance. Nonrnagnetic alloys couldalso be used to obtain the high resistance needed, but higher resistancein a metal is usually associated with higher thermal resistance and,therefore, lower power capacity as limited by the heating permissible. Amagnetic material thus overcomes this limitation and provides highsurface resistance with high thermal conductivity.

The high thermal conductivity of the complete attenuator is a veryimportant feature of the present invention. The metal washers 1i) and 12in FIG. 3 are the elements in which the heat is introduced by thedissipation of electromagnetic energy. These same elements physicallyextend to the outer circumference of the attenuator without interruptionor discontinuity. Therefore, as heat is generated, it is rapidlyconducted outward where such heat can be very rapidly removed by the useof cooling fins, cooling water flowing through a surrounding jacket,etc. The provision of such external cooling means is well known and neednot be discussed in detail. One example will be dealt with briefly inconnection with FIG. 5.

Use of the dielectric washers such as 16 is optional, according to therequirements of physical strength, cost, etc. Obviously, for very thinmetal washers, dielectric washers may be needed to provide physicalrigidity. For extremely high power, the temperature rise may require theuse of dielectric washers constructed of mica or ceramic. The inclusionor exclusion of the dielectric washers is thus not a factor which can bespecified for all possible applications, and is, therefore, best left tothe designer facing a particular application.

Referring now to FIG. 5, there is shown a longitudinal bisected view ofa termination for a coaxial transmission line. The same referencenumerals as in FIG. 1 have been used where appropriate. As shown, theconstruction is basically the same as for the attenuator of FIGS. 1-3,except that, since this termination is designed to totally dissipate aninput electromagnetic wave, there is no output port. It will beappreciated that, since perfection is not attainable, there will alwaysbe some reflection coefficient or standing-wave ratio associated withthis device, and the best that can be accomplished is substantiallytotal dissipation of an input wave. In order to improve impedancematching in the FIG. 5 termination, the center conductor is tapered sothat the spacing between the Washers 10, 12, etc., and the centerconductor 20 varies. As shown, the complete active termination is ahalf-wave long, all such measurements in this specification beingspecified on the basis of a compromise wavelength chosen within theintended operating frequency range of the device. The actual dimensionsof the washers are chosen so that the zigzag path length, times the unitsurface resistance, provides a total resistance sufiicient to totallydissipate an input wave.

As shown, the FIG. 5 termination has been constructed so as to be verynearly matched to the line over a very wide frequency band (such as aratio of 4 to 1). The washers in the outer conductor are designed topresent a value of total surface resistance equal to the line impedance,then the inner conductor is tapered gradually to maintain at every pointa match to the line impedance at that point. This taper is arrived atusing known transmission line impedance calculating techniques, and therequired taper is close to the exponential form approximated in FIG. 5.This principle could obtain a perfect match down to zero frequency,except that the surface resistance varies slowly with frequency.Therefore, a close match can be obtained to the extent of the bestcompromise over any specified band. In other attenuators, the centerconductor may incorporate stepped quarter-wave sections of differentdiameter in place of the taper as shown. The provision of steppedimpedance matching sections is well known.

Also included in FIG. 5 is a surrounding hollow jacket having an innerwall 22, in good thermal contact with the outer circumferences of allthe metal elements 10, 12, 14, etc., a spaced outer wall 24, an inputport 26, and an output port 28. As shown, the walls 22 and 24 are formedintegrally with bridging end sections so that an annular circumferentialchamber is formed around the complete termination. In operation, acooling fluid such as water can be circulated through the chamber, viaports 26 and 28, to provide rapid heat removal. Cooling fluid chambers,fins to permit cooling by air convection, or other arrangements forremoving heat can be provided for use with all attenuators constructedin accordance with the invention.

FIGS. 6, 7, and 8 are three views of a termination for use withrectangular waveguide. As shown, iron sheets 30 in stacked spacedrelation form the narrow side walls of a waveguide section. These sheetshave their main surfaces parallel to the broad walls 32, and the outerextremities of the sheets are in common electrical contact at 34. Asshown, the active portion of the termination is a half-wavelength long,and a quarter-wavelength section of diminished height is provided forimpedance matching purposes. At 36 is shown a standard waveguide fiangepermitting coupling to a standard rectangular waveguide section as.

As is well known in waveguide theory, transmission of electromagneticwaves in waveguide causes current to flow vertically in the side wallsof the waveguide. With construction such as shown in FIGS. 6, 7, and 8,such currents are made to travel a long zigzag path similar to thatdescribed with reference to FIGS. 1-4. Dissipation of energy resultsfrom providing a long path in material, like iron, having a high surfaceresistance. It will be understood that the Well-established principlesof transmission line and waveguide theory and practice are applicableand quarter-wave stepped sections, tapered sections, etc., for impedancematching can be provided by persons skilled in the art using suchestablished principles.

A termination constructed in accordance with the invention andresembling FIG. 5 was built and successfully tested. Such terminationhad the following dimensions and provided the test results listed below:

Length 4 inches.

Washer material Iron.

Spacer material Mica.

Washer inner diameter /2 inch.

Washer outer diameter /2; inch.

Washer thickness 0.002 inch.

Spacer thickness 0.002 inch.

Peak power 3 0 kw.

Average power lkw.

Frequency 1,300 mc. per second.

VSWR Less than 1.3.

Washer permeability Approx. 50.

Washer skin depth Approx. 0.1 micron or 4 microinches.

In accordance with the present invention, energy dissipation isaccomplished directly in iron sheets for example. The major significanceof this fact is that high power capability results from the combinationof good thermal conduction and high permissible operating temperatures.Thus, the power rating per unit size is increased greatly over priordesigns which were of limited power rating, due primarily to theinability to provide an unbroken path of high heat conductivity directlyto the outside of an attenuator.

While there has been described what is, at present, considered to be thepreferred embodiment of the present invention, it will be obvious tothose skilled in the art that various changes and modifications may bemade therein Without departing from the invention; and it is, therefore,aimed to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

What is claimed is:

1. An attenuator comprising:

a plurality of spaced iron sheets in common electrical contact at theouter edges with the central portions spaced and noncontacting andhaving openings in the central portions cooperating to permitpropagation of electromagnetic waves;

the attenuator being so constructed and arranged that currentsassociated with such waves are made to travel a long zigzag path alongthe surface of each sheet out to the common contact, back in again alongthe surface of the next sheet, etc., and the surface resistanceencountered along said path dissipates energy while the resulting heatis conducted away through the sheets.

2. An attenuator in accordance with claim 1 which includes an adequatenumber of sheets to provide a current path with a total surfaceresistance sufficient to substantially totally dissipate anelectromagnetic wave coupled to said attenuator, whereby such wave willbe terminated.

3. An attenuator in accordance with claim 1, wherein said sheets areconstructed of iron having high permeability and high thermalconductivity and the thickness of each iron sheet is more than two timesthe skin depth of current in the operating frequency range.

4. An attenuator comprising:

a plurality of laterally spaced circular iron washers:

in common electrical contact at the outer circumference with the centralportions spaced and noncontacting and having coaxial central openingscooperating to 'permit propagation of electromagnetic waves;

the attenuator being so constructed and arranged that currentsassociated with such waves are made to travel a long zigzag path alongthe surface of each washer, out to the common contact, back in againalong the surface of the next washer, etc., and the surface resistanceencountered alon said path dissipates energy while the resulting heat isconducted away radially through the washers.

5. An attenuator in accordance with claim 4 which additionally comprisesa central conduct-or coaxial with said central openings to form acoaxial transmission line attenuator.

6. An attenuator in accordance with claim 5, wherein said washers areconstructed of iron having high permeability and high thermalconductivity, and the thickness of each washer is more than two timesthe skin depth of current in the operating frequency range.

7. An attenuator in accordance with claim 5, wherein the spaces betweensaid iron washer are filled by dielectric washers having centralopenings similar to the openings in the metallic washers.

8. An attenuator in accordance with claim 5, wherein the spacing betweenthe center conductor and the iron washers is exponentially tapered forproviding impedance matching.

9. A coaxial transmission line termination comprising:

a plurality of laterally spaced iron washers with metallic bridgingsections at the outer circumferences effectively forming a continuousmetallic cylinder surrounding the central portions of the washers whichare spaced and noncontacting and have coaxial openings cooperating topermit propagation of electromagnetic waves, the thickness of the ironwashers eing more than two times the skin depth of current in theoperating frequency range;

a central conductor passing through said central openings coaxially;

the termination being so constructed and arranged that whenelectromagnetic waves are coupled to the termination, the resultingcurrent in the outer conductor is made to travel a long zigzag pathalong the surface of each washer out to the circumferential bridgingsections, back in again along the surface of the next washer, etc., andthe surface resistance encountered along said path is suflicient tosubstantially totally dissipate said waves.

10. A termination in accordance with claim 9, wherein the spaces betweensaid iron washers are filled by dielectric washers having centralopenings similar to the openings in the iron washers.

11. A termination in accordance with claim 9, wherein the spacingbetween the center conductor and the iron washers varies for providingimpedance matching.

12. A waveguide attenuator comprising:

a plurality of spaced iron sheets whose main surfaces are parallel tothe direction of propagation with the outer extremeties of the sheets incommon electrical contact and the inner portions noncontacting andcooperating to define a waveguide permitting an electromagnetic wave topropagate in the space bounded by the inner edges of the sheets;

the attenuator being so constructed and arranged that currentsassociated with waves propagating in said attenuator are made to travela long zigzag path along the surface of each sheet out to the commoncontact, back in again along the surface of the next sheet, etc., andthe surface resistance encountered along said path dissipates energy.

13. A waveguide attenuator for use with rectangular waveguidecomprising:

a waveguide section wherein each narrow wall of the waveguide comprisesa plurality of spaced iron sheets whose main surfaces are parallel tothe broad walls of the waveguide with the outer extremeties of thesheets in common electrical contact and the inner portions spaced andnoncontacting, the metal sheets of the two narrow walls cooperating toform a waveguide permitting electromagnetic waves to propagate in thespace bounded by the broad walls of the waveguide and the inner edge ofthe metal sheets;

the attenuator being so constructed and arranged that currentsassociated with waves propagating in said attenuator are made to travela long zigzag path along the surface of each sheet, out to the commoncontact, back in again along the surface of the next sheet, etc., andthe surface resistance encountered along said path dissipates energy.

14. An attenuator in accordance with claim 13, which includes anadequate number of sheets to provide a current path with a total surfaceresistance sufficient to substantially totally dissipate anelectromagnetic wave coupled to said attenuator, whereby such wave willbe terminated.

15. An attenuator in accordance with claim 13, wherein said sheets areconstructed of iron having high permeability and high termalconductivity, and the thickness of each iron sheet is more than twotimes the skin depth of current in the operating frequency range.

16. An attenuator in accordance with claim 13, wherein said waveguidedefined by the inner edges of the metal sheets diminishes in heightalong the direction of propagation for providing impedance matching.

References ited by the Examiner UNITED STATES PATENTS 2,556,642 6/1951Bird 33322 2,653,270 9/1953 Kompfner 33331 X 2,761,828 9/1956 Eldredgeet al. 33331 X 2,779,006 1/1957 Albersheim 33381 X 2,804,598 8/1957 Fano333-22 3,041,558 6/1962 Brown et al. 333'22 X 3,050,606 8/1962 Tibbs3338l X 3,083,528 4/1963 Brown 33322 X 3,158,824 11/1964 Larsen et a1333* 3,205,459 9/1965 Ada-ms 333-22 OTHER REFERENCES Southworth:Principles and Applications of Waveguide Transmission, copyright 1950,Van Nostrand, New York, page 380 relied on.

HERMAN KARL SAALBACH, Primary Examiner. R. F. HUNT, Assistant Examiner.

1. AN ATTENUATOR COMPRISING: A PLURALITY OF SPACED IRON SHEETS IN COMMONELECTRICAL CONTACT AT THE OUTER EDGES WITH THE CENTRAL PORTIONS SPACEDAND NONCONTACTING AND HAVING OPENINGS IN THE CENTRAL PORTIONSCOOPERATING TO PERMIT PROPAGATION OF ELECTROMAGNETIC WAVES; THEATTENUATOR BEING SO CONSTRUCTED AND ARRANGED THAT CURRENTS ASSOCIATEDWITH SUCH WAVES ARE MADE TO TRAVEL A LONG ZIGZAG PATH ALONG THE SURFACEOF EACH SHEET OUT TO THE COMMON CONTACT, BACK IN AGAIN ALONG THE SURFACEOF THE NEXT SHEET, ETC., AND THE SURFACE RESISTANCE ENCOUNTERED ALONGSAID PATH DISSIPATES ENERGY WHILE THE RESULTING HEAT IS CONDUCTED AWAYTHROUGH THE SHEETS.